A general method of forming an active layer of a solar cell using a wet process mainly includes: dissolving an active layer material, e.g., PTB7:PC71BM, in a solvent to form an active layer solution; and coating the active layer solution on a substrate to form an active layer using a spin coating technique, a sheet-to-sheet process, or a roll-to-roll process. To improve photovoltaic conversion efficiency of the solar cell made from conductive polymers with low band gap, an additive having a relatively high boiling point (e.g., 1,8-iodooctane (DIO) or 1-chloronaphthalene (CN)) is added into the active layer solution. Thus, the active layer formed has a superior internal nanostructure that facilitates carrier transmission, separation of the conductive polymers and fullerenes, and crystallization of the conductive polymers.
The active layer solution coated on the substrate using the spin coating technique is dried at a relatively fast speed under the room temperature. For a batch process, such as the sheet-to-sheet process or the roll-to roll process, the active layer solution that is coated on the substrate needs to be dried to form the active layer. However, an undesirable flow of the active layer solution is likely to occur on the substrate before the active layer solution is dried, and may result in a non-uniform distribution of the additive in the active layer, which may increase deviation of the power conversion efficiency (PCE) of the solar cell.
In order to overcome the aforesaidproblem, the active layer may be heated to expedite drying so as to prevent the undesirable flow. However, the heating process may adversely affect batch-to-batch consistency of the power conversion efficiency of the solar cell.